When organs of the body fail, various toxins accumulate in the body while other chemicals become deficient. When organ failure is severe, the patient becomes ill. To support the patient, one option is to remove blood and pass it through an extracorporeal blood therapy (EBT) instrument. The instrument typically includes a membrane unit for diffusing or convecting toxins out of the blood into a dialysate fluid or into solid absorbants which selectively absorb the toxins. These extracorporeal blood therapy instruments include hemodialysis and hemofiltration machines to treat kidney failure, heart-lung machines, hemoprofusion columns, plasmapheresis machines, and cell separators. Most extracorporeal blood therapy instruments use some type of pump such as a roller pump for removing blood from a vessel through a needle or catheter, propelling it through the membrane unit, and returning it to the body. Creating adequate blood flow through the instrument would seem simple but, in fact, it is complicated by a number of problems. First, the blood flow rate is variable depending upon the blood flow within the cannulated vessel, the internal diameter of the needle or catheter, the viscosity of the blood, the internal diameter of the vinyl tubing typically used with a roller pump, and the revolution rate of the pump. Furthermore, roller pumps are not volumetric. When resistance increases on the input or output line, blood flow rate drops even though the revolution rate of the pump remains constant.
A second problem occurs when the tip of the intravenous needle or catheter pulls the vein wall against the opening therein, suddenly stopping blood flow.
A third problem is when negative pressure created by the blood pump causes air to leak into the instrument, thereby causing foam or bubbles within the blood. The return of air bubbles to the patient could be life-threatening.
Artificial materials making contact with the blood tend to create blood clotting, thus increasing flow resistance through the instrument.
Another problem occurs in manually "priming" the instrument. This necessarily includes inversion of the membrane unit to purge air from the blood and dialysate sides of the membrane unit. This requires careful observation on the part of the operator to assure that all air bubbles have been removed before use of the instrument.
To assure that blood flows through a blood therapy instrument at a prescribed rate, pressure gauges have been previously attached to the upper portion of drip chambers. These drip chambers allow air to separate from the blood and stay in the upper portion of the chamber while blood exits from the bottom portion of the chamber. A change in pressure measured by these gauges indicates a change in flow through the instrument. Alarms for adverse blood flow conditions occur only when the operator has manually set pressure limits close to initial venous and arterial pressure levels when blood is flowing properly. In the absence of these alarms, the operator must manually observe the flow of blood in the tubing to and from the roller pump.
Dialysis operators spend considerable time observing the flow of blood through the tubing and roller pump during routine hemodialysis of patients. This operator observation during initial priming of the instrument with saline is required, because current dialysis monitor devices cannot accurately perform precise blood flow rate measurements and rapid detection of inflow obstructions. Furthermore, these devices cannot determine the number of microbubbles in the inflow blood lines before an air bubble with significant risk is produced, detect air bubbles during priming to indicate when air is completely replaced by priming fluid, and determine the quantity of platelet/fibrin aggregates leaving the blood therapy instrument to detect dialyzer clotting at a time when increasing anticoagulant would prevent further clotting.
In addition to the above problems, there are a number of problems associated with fluid replacement during extracorporeal blood therapy. The unidirectional movement of blood through a therapy instrument is simply and directly performed by a roller pump, in which rollers move circularly and compress a resilient cylindrical tube. This motion draws blood through one needle or catheter lumen from a patient, propels it through the tubing and membrane package, and returns it to the patient through another needle or catheter lumen. In reality, though, the tubing set is more complicated than this. Blood is not the only fluid which must be delivered to the patient. If the patient's blood pressure falls, several hundred cubic centimeters (cc) of saline or colloid solution must be administered. At the end of the dialysis, a similar amount of saline is infused to rinse blood from the dialyzer. During dialysis, the need for an anticoagulant is diminished by rinsing blood from the dialyzer with 100 ml of saline each one half hour, the "low heparin rinse" schedule. Finally, many blood products and medications are most easily and effectively delivered intravenously during dialysis.
In present dual-needle blood therapy procedures such as hemodialysis, there are a number of manual methods for effectively delivering sterile fluid into blood within the therapy instrument. However, these methods have several significant problems. The negative pressure in the arterial line is variable, thus producing a variable fluid flow rate. The amount of delivered fluid depends upon the flow rate and time of infusion, both of which are not exactly known. Furthermore, there is no device to measure the exact amount of fluid delivered to the patient. Simply observing the change in the air-fluid interface within the fluid bag is an inaccurate method for determining the volume delivered. Without close observation, excess fluid can be drawn into the blood circuit, resulting in fluid overload of the patient. Further, if the fluid container empties, air passes into the arterial line and into the dialyzer. If a large volume of air enters the membrane unit, it cannot be quickly or easily removed, and the dialysis procedure must be aborted.